1. Field of the Invention
The present invention relates to a reflective-type liquid crystal display apparatus, and relates to a reflective-type liquid crystal display apparatus which is preferably applied to an office automation system, a personal computer, a personal digital assistant, a cell phone and the like.
2. Description of the Related Art
As multimedia progresses rapidly in society, various kinds of information media are being considered for implementation as electronic devices. One case thereof is information whose medium is paper such as printed matter, for which it would be desirable to develop a display apparatus in the form of electronic paper, for example. This display apparatus is as thin as paper and can be elastically deformed as necessary. Furthermore, this display apparatus is required to be capable of high definition display of characters and charts at the same level as printed matter. One type of display apparatus that has been considered is the use of a reflective-type liquid crystal display apparatus of an active matrix driving system, in which plastic or a resin film being elastically deformable is used as a substrate and a voltage applied to a liquid crystal layer is controlled by a switching element.
In the reflective-type liquid crystal display apparatus, a switching element used mainly is a thin film transistor element (a TFT element) and a metal insulator metal element (an MIM element), which is a thin film two-terminal element. Since, of these switching elements, a TFT element is normally produced at a temperature of approximately 300° C. or more, it is difficult to use plastic or a resin film as a substrate because a heat-resistance temperature thereof is limited. On the other hand, since a production temperature of a thin film two-terminal element is set to approximately 180° C. or less, a limitation of a heat-resistance temperature thereof is loosened largely as compared with in that of the TFT element. Therefore, as disclosed in Japanese Unexamined Patent Publications JP-A 6-214220 (1994), JP-A 8-271932 (1996) and JP-A 8-286198 (1996), it is possible to use plastic or a resin film as a substrate, and it is studied to use a reflective-type liquid crystal display apparatus having an elastically deformable substrate in various ways.
FIG. 15 is a plan view showing one pixel of a reflective-type liquid crystal display apparatus in which a conventional thin film two-terminal element is formed. In production of an element-side substrate of the reflective-type liquid crystal display apparatus, a tantalum film, that is, a Ta film is formed on an insulating resin substrate 1 firstly, and thereafter, a first conductor layer 2 to become wiring and a lower-layer electrode is formed by photolithography and etching. Secondly, the surface of the first conductor layer 2 is anodized, and a nonlinear resistance film is formed. Next, after a titanium film is formed on the resin substrate 1, a second conductor layer 3 to become an upper-layer electrode is formed by performing photolithography and etching again. After that, an aluminum film is formed on the resin substrate 1, and then, a third conductor layer 4 which becomes a pixel electrode and a reflection layer is formed by performing photolithography and etching again.
In the case of using the resin substrate in a reflective-type liquid crystal display apparatus, the deformation ratio of the substrate in processing is, for example, approximately 10 to 1000 ppm, and for allowing this, there is a need to ensure an alignment margin of approximately 10 μm or more. On the other hand, in the case of using a glass substrate in a reflective-type liquid crystal display apparatus, the deformation ratio of the substrate in processing is approximately 10 ppm or less, and a design margin for allowing deformation at alignment is designed with accuracy of, for example, approximately 10 μm or less. In the case of a panel having approximately 4 inch diagonal lines which is approximately 8 cm in a row direction (R direction) and approximately 6 cm in a column direction (C direction) in size, assuming that the deformation ratio of a resin substrate in processing is, for example, 300 ppm, values of size deformation of the substrate is 24 μm in the row direction obtained by multiplying the deformation ratio 300 ppm and the row-direction size 8×10−2 m together and 18 μm in the column direction obtained by multiplying the deformation ratio 300 ppm and the column-direction size 6×10−2 m together.
In a production method of a substrate of Japanese Unexamined Patent Publication JP-A 3-46632 (1991), a technique of enabling reduction of a region necessary for ensuring an alignment margin is disclosed. In the production method, lower electrode wiring and a reflective pixel electrode are formed by the use of a reflective metal film, and an upper electrode is combined with it, whereby a thin film two-terminal element is formed by two exposure processes in all.
The prior art of using plastic or a resin film as a substrate has the following problems. That is to say, a plastic or resin film substrate to become an insulating substrate constituting a liquid crystal display apparatus has a lower heat-resistance temperature than a glass substrate. Furthermore, resulting from changes in temperature and humidity, a change in size, that is, deformation such as extension or shrinkage tends to appear in the substrate. Moreover, there is a problem that the substrate is warped and deformed by stresses from various kinds of thin films formed on the substrate. For example, according to pages 6 to 8 of Electronic Engineering July, 2000, the linear expansion coefficient of a resin film substrate used in a liquid crystal display apparatus is approximately ten times the linear expansion coefficient of a glass substrate. In consequence, by a change in temperature of, for example, 1° C., the size of the substrate is changed, that is, the substrate is deformed approximately ten times. Therefore, in a production process of an active element which needs an accurate alignment process such as a thin film two-terminal element, it is difficult to obtain sufficient accuracy of the size of a substrate. In a case where an alignment margin is set largely to accommodate for deformation of a substrate, alignment of various kinds of thin films to the substrate is enabled, whereas an aperture ratio, that is, the ratio of an actual active screen area to a display screen area of a liquid crystal display apparatus is decreased. In consequence, not only a display character is degraded, but also a sufficient margin is not ensured in designing a minute pixel for high definition display.
In the case of using the resin substrate 1 in a reflective-type liquid crystal display apparatus as shown in FIG. 15, it is necessary to set an alignment margin α in the row direction of a relation between the first conductor layer 2 and the third conductor layer 4 and an alignment margin γ in the column direction of a relation between the first conductor layer 2 and the second conductor layer 3 to 24 μm, respectively, and it is necessary to set an alignment margin β in the column direction of a relation between the first conductor layer 2 and the third conductor layer 3 and an alignment margin δ in the column direction of a relation between the second conductor layer 3 and the third conductor layer 4 to 18 μm, respectively. As a result, the aperture ratio of the reflective-type liquid crystal display apparatus is decreased, and the display character thereof is degraded. Besides, since the alignment margins in the column direction and the row direction become large as the panel size becomes large, it is necessary to make the size of a pixel large enough to ensure necessary margins, and therefore, high definition display is impossible.
According to the production method disclosed in JP-A 3-46632, an exposure process in which alignment is required is executed only once, so that it is possible to reduce regions necessary for ensuring alignment margins. A metal film which forms an electrode constituting a thin film two-terminal element and a reflective pixel electrode can be made of a material such as tantalum Ta or aluminum Al. However, as for tantalum Ta, an element characteristic of a thin film two-terminal element using an anodized film of tantalum Ta is sufficient, whereas as a reflective pixel electrode, the reflectance thereof is approximately half of that of aluminum Al used in general, and a sufficient characteristic cannot be obtained. In the case of using aluminum Al, a performance of a reflective pixel electrode can be obtained, whereas a thin film two-terminal element using an anodized film of aluminum Al cannot have a requisite element characteristic. In consequence, it is impossible to obtain characteristics required for a thin film two-terminal element and a reflective electrode, respectively, at the same time, and it is necessary to sacrifice either characteristic.
FIG. 16 is a perspective view showing, by cutting away, a substantial part of a conventional reflective-type liquid crystal display apparatus 5 disclosed in Japanese Unexamined Patent Publication JP-A 6-235940 (1994). In this reflective-type liquid crystal display apparatus 5, a first conductor 6 of an MIM element constituted by a nonlinear resistor 7 and a second conductor 8 is made to be signal wiring, the second conductor 8 is made to be a pixel electrode and a reflector, and the nonlinear resistor 7 and the second conductor 8 are formed on the first conductor 6, so that a site for only the signal wiring and a site for only a thin film two-terminal element are not required. Therefore, it is possible to make the spacing of pixels narrow, so that it is possible to make the spacing of pixels narrow and make the area of a pixel large, and it is possible to increase an aperture ratio. However, since the nonlinear resistor 7 is placed in the same region and with the same area as the second conductor 8, there is a problem that it is difficult to ensure alignment margins when overlaying and forming the first conductor 6, the nonlinear resistor 7 and the second conductor 8.
Although the above problems are tasks in using plastic or a resin film for a substrate, in a case where it is desired to develop a higher definition display apparatus, an alignment margin at the time of production and an aperture ratio are of a problem also in a glass substrate.